I have three tanks two 10's and a 55 and have had them sat up for several months with no problems and a couple of weeks ago i noticed an alge build up starting so i sucked them down 50% and pulled out all the fixtures (plastic plants rocks,etc) and gave them a good scrubbing and vacumed out the gravel real good and in less than a week it is back with a vengance how do i get rid of this crap without killing my fish.